Box Select Error: Active Object is Not a Selected Mesh
No, the message is right. And happens also selecting by hand, I just did it (blender 2.74 on this pc)

In blender, the "active object" is the last one selected.

But. suppose you have 3 objects and


You start selecting obj1: that is the active object, and is also selected
then you shift select obj2 and obj 3: now obj1,2,3 are all selected but obj3 is the active (last selected)
then you (eg: by mistake), click again on obj1: now obj2,3 are selected, obj1 is not BUT it is the last you selected (and then deselected) so, to blender obj1 is still the "active" object, but is not selected 


exactly what the message says...

In your image, I see that in the outliner this is visible. the active object (not selected) is backpole.003 (its text is highlighted but the triangle icon has no circle background) while backpole.009 and backpole.019 are selected (the triangle icon has a circle purple background)

If you ask me, it could be a bug in the selection behaviour, but... this explains the message you get. It could be that box selection replicates this behaviour or something similar with the same effect